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FMCOMMS5 GPS Satellite Frequency Shift

Question asked by odabak on Feb 28, 2017
Latest reply on Mar 6, 2017 by Vinod

Hello,

 

In our project, we want to use ZC706 and FMCOMMS 5 as a GPS receiver. 

 

For GPS application, the center frequency is 1575.42 MHz. Due to Doppler effect, the center frequency of the satellites can shift +- 10 kHz from the 1575.42 MHz. In GPS acquisition algorithm, the search space for the satellites are determined according to this shift.

While we are working, we notice that the frequency shift was between 10 kHz and 15 kHz. Therefore we are suspicious about the LO frequency of the FMCOMMS5.

In our work, we have set the center frequency of the LO to 1575.42 MHz and we downconvert the signal to baseband.

In FMCOMMS5, the crystal oscillator Rakon 513371 is used as an external clock of the AD9361. There is no such an error of this clock(40 MHz clock, 25ppm, +-1kHz).We don't expect our error from this clock. In addition to that, We have created a CW signal around 1576 MHz with 1575 MHz LO frequency and observed the shift of CW signal's frequency and we couldn't see such a big shift above 1 kHz.

In conclusion, we have observed unexpexted shift in center frequency of the satellites but we almost sure about the accuracy of the reference clock and LO set.

Can you make a comment about the other potential reasons of this situation?

 

Thanks to all,

 

Ömer 

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